The present invention relates to ceramic substrates for IC packages used in single layer flat packages, multi-chip mounting substrates, pressure sensor substrates, and the like, and particularly to a substrate for an IC package in which no grinding is required to fabricate the substrate.
Conventionally, a substrate for an IC package (hereinafter referred to as "IC substrate") to be used in a single layer flat package, a multi-chip mounting substrate, a pressure sensor substrate, and the like, has been manufactured through the following steps:
(1) Comminuting, mixing and drying the raw materials; PA1 (2) Molding the mix; PA1 (3) Sintering; PA1 (4) Grinding; and PA1 (5) Metallizing a portion of the device.
The quality of the IC substrate is affected by all the fabrication steps. Such devices are particularly affected by the grinding step, as well as the succeeding metallizing step. This is because an electrically conductive film is formed on the ceramic IC substrate in the configuration of a circuit. Therefore, the degree of smoothness (microscopic unevenness) and the degree of flatness (macroscopic unevenness) on the surface of the conductive film is critical in order to keep the accuracy of the circuit.
Such a requirement is particularly significant where the IC substrate receives a metallized film through the formation of a thin film by a process such as vacuum evaporation, sputtering, or the like. Because the film formed through vacuum evaporation or sputtering has a thickness of about 1 .mu.m, the surface roughness of the substrate is required to be 0.1 to 0.2 .mu.m, that is one order of magnitude lower than the thickness of the film. Early in the development of such devices, slide glasses for a microscope were substituted for thin film IC substrates. Such slide glasses have a smooth ground surface and were available at a low cost. Later, glass substrates intended for such a use were produced. Alumina substrates having glazed surfaces and having superior mechanical and thermal characteristics have also been produced in a quest for more highly reliable substrates. However, even those substrates were not completely adequate in the desired characteristics, and alumina substrates coated with a very thin glass layer or alumina substrates with their surfaces ground smooth have been used as improved substrates.
The manufacturing of such glass-coated alumina substrates involves several additional steps that reduce production rates. Further, in the case where the surfaces of the alumina substrates are ground smooth, there are disadvantages because the surface particles may come off, impurities may be mixed thereinto, and so forth.
In order to solve these problems, alumina substrates which can be metallized without prior grinding have been developed by using high purity alumina. However, such materials have the disadvantage that the sintering temperature is high, the dielectric constant and the coefficient of expansion are relatively large, and the manufacturing cost is excessive.